1. Field of the Invention
The process of the invention relates to a process for the separation of organic phosphines and/or phosphine oxides from solvents such as are used for the production of propylene oxide and especially to the recovery and recycle of organic phosphines and/or phosphine oxides in a propylene oxide production process.
2. Background of the Invention
A great deal of effort has been expended on the development of a no coproduct direct oxidation process for the production of propylene oxide. U.S. Pat. No. 6,005,123 is illustrative of the encouraging results which have been achieved.
As reported in U.S. Pat. No. 6,005,123, propylene oxide can be prepared by reacting propylene, oxygen and hydrogen in the presence of a noble metal on titanium silicate catalyst, e.g. Pd on TS-1. Usually the solid catalyst is slurried in an appropriate solvent such as a methanol and water mixture during the reaction.
Especially advantageous results are achieved where reaction promoters of the phosphine or phosphine oxide type such as triphenyl phosphine are also present in the propylene oxide forming reaction mixture.
To enable phosphines and phosphine oxides to be used in a continuous chemical process they must be recycled. The use of phosphines and phosphine oxides is prohibitively expensive if the phosphines and phosphine oxides cannot be conveniently recycled for reuse. Unfortunately, a process in which methanol and/or water are used as solvents renders most of the typical unit operations unusable. As methanol is distilled, or stripped, from the reaction mixture the phosphines and phosphine oxides precipitate from solution, thus fouling the stills. Furthermore, the phosphines and phosphine oxides are not stable enough, or volatile enough, to distill overhead. Even if the water is somehow distilled overhead, while keeping the phosphine and phosphine oxides in a solution of PEG and other propylene oxide ring opened products, the separation of the ring opened products from the phosphine and phosphine oxides cannot be accomplished by distillation. Hence, distillation will not work.
Compounding the fact that distillation is not an option is that extraction technology will not work efficiently, either. Even if one could obtain the phosphine and phosphine oxides in a solution of ring opened products, solvents that will solvate the phosphines and phosphine oxides are miscible with the ring opened moieties. Even if a solvent could be found which could selectively extract the phosphines and phosphine oxides from the ring opened solution, this process would require more distillation operations in order to recycle the extraction solvent.